The primary objectives of a transceiver antenna are to: (1) allow quick and convenient radio communications between radio communications stations; (2) obtain acceptable transmission and reception performance gains within operating design parameters; and (3) selectively reject noise or invalid input or output signals. It also must be capable of tolerating changes in direction or frequency and be very reliable. It should also be low profile (to reduce air drag) light weight (to minimize structural support), easy to maintain, rugged in construction, pleasing in appearance and low in cost. When the antenna is used in each of the two operating modes (receiving and transmitting), a minimum of effort to convert from one mode to another mode is also desirable. If the antenna is to be used on a mobile installation, it must also be able to perform in a difficult environment and with the power, space and other limitations of the vehicle or other mobile installation.
Typical earth bound mobile transceivers currently employ quarter-wave whip antennas (see: Standard Handbook for Electrical Engineers, Tenth Edition, by Donald G. Fink and John M. Carroll, editors, McGraw-Hill, 1968, New York, page 25-74). A fairly uniform omnidirectional vertical polarity pattern is obtained from such installations. However, these antennas are low gain antennas which will not meet current satellite land mobile and airborne communication system requirements. In addition, these low gain antennas are subjected to multipath interference effects.
The use of dipole elements in an antenna can be as simple as a straight radiator fed in the center to produce currents with two nodes, one at each of the far ends of the radiator (see Van Nostrand's Scientific Encyclopedia, Fourth Edition, D. Van Nostrand Company, Princeton, N.J., 1968, Page 537). In order to improve tuning and balance, various geometries are used. Two separate radiator elements can also be used. Variations with two separate radiator elements include: altering the geometries of the nodes, folded radiators and adding/altering the dielectric between the elements.
The single slot type of antenna is a variation of the basic dipole antenna (see the section on Slot Antennas, specifically the relationship to metallic dipole antennas, Reference Data for Radio Engineers, Fourth Edition, Published by International Telephone and Telegraph Corporation, New York, 1956, pages 687-689, and U.S. Pat. No. 3,210 766). Each side of the slot acts as one node of an elementary dipole. The length and separation dimensions of the slot are selected to maximize performance (ie: a fraction of a normal wavelength).
In order to obtain directionally selective antenna performance, an array of slotted antennas can be formed, with each antenna in the array fed a phase controlled signal which will cancel (be out of phase) signals from adjoining antennas within the array at certain directions, and amplify (be in phase) adjoining signals in other selected directions (see U.S. Pat. Nos. 4,340,891, and 3,969,730). A plurality of slots in different orientations (either with or without an array) is also known (see U.S. Pat. No. 4,245,222). Each of the plurality of slots is excited or coupled by a feeder for each slot. Sometimes the various slots receive phase shifted signals from feeds located at opposite ends of the radiating element (see U.S. Pat. No. 3,720,953). These feeds are connected either to each side of the conductive element or to each slot.
Printed circuit slotted antennas are also known (see "Printed Dipoles Electromagnetically Coupled To Coplanar Waveguide For Linear Or Circular Polarization", by R. W. Jackson, published in Electronic Letters, The Institution Of Electrical Engineers, Mar. 13, 1986, Vol 22, No. 6). Again, a single feed is used for each slot.
Four feeders exciting the intersection or center of the cavity is also known (see "A Shallow Ridged-Cavity Crossed-Slot Antenna for the 240- to 400-MHz Frequency Range", by H. E. King and J. L. Wong, published in IEEE Transactions on Antennas and Propagation, September 1975, Pages 687-689, and "A Shallow-Cavity UHF Crossed Slot Antenna", by C. A. Lindberg, published in IEEE Transactions on Antennas and Propagation, September 1969, Pages 558-563. These prior art examples connect four feeds to the center or intersection of the slots. Each corner of the intersection is connected with a phase shifted signal.
Several attempts have been made to develop a low profile, high gain phased antenna array to achieve acceptable radio communication between a mobile unit and a satellite, based on printed-circuit crossed-slot antennas which have been built and tested (Proceedings of the Mobile Satellite Conference, May 3-5, 1988, Jet Propulsion Laboratory, California Institute of Technology, Pasadena Calif., pages 16 and 21). A crossed-slot element having a two point feed (one pair of conductors for each crossed slot) was originally designed and tested for this application (Antennas and Propagation, Volume 1, June 15-19th, 1987, Virgina Polytechnic Institute & State University, Blacksburg, Virginia, IEEE Antennas and propagation Society, paper entitled MSAT-X Phased Array Cross-Slot Element Design, by the inventors, page 357). The prior art of printed-circuit crossed-slot design was expected to minimize the need for a plurality of separate slots feed circuits in an array.
However, testing of this dual feed crossed slot design showed good impedance match over an acceptable bandwidth but inefficient radiation gain performance (page 357, supra). Further analysis (page 357, supra and supra Volume 11, paper entitled "Analysis of Low Cost Conformal Phased Array for the MSAT-X Applications", by one of the inventors, H. H. Chung, pages 1156-1159) and testing showed significant coupling of the radio frequency signals from one of the crossed slot legs to another.
Limitations of these prior art crossed slot antennas are primarily related to the above mentioned loss of performance and the poor gain produced within the bandwidth. Typical gains of a two point feed crossed-slot antenna are in the order of -1.0 to 0.0 dBic.
No prior art that the applicant is aware of discloses two signal stripline feeds with a 180 degree phase shift for each printed slot of the printed cross-slot.